I do not suffer from jet lag that much but I have friends who suffer terribly. I ascribe my greater tolerance to my belief that a prime cause is tiredness during the long flight and so make it a point to sleep as much as possible on the plane, which I am fortunately able to do. Some people find it very hard to sleep and watch a lot of inflight films which may make the even more tired. I have also experienced that traveling west is easier than traveling east and I put that down to the fact that going west results in the day-night cycle becoming stretched out and so one has longer nights and can more sleep.
But it turns out that there is more going on and an article in the October 2016 issue of Physics Today (p. 23) summarized a study that explained that jet lag is caused by a network of 20,000 neurons in a region of the brain known as the suprachiasmatic nucleus (SCN) that main the body’s circadian rhythm falling out of sync with the local daily day-night cycle and that this leads to large recovery times.
Although the neurons all take cues from the same source—the retina—they don’t all respond in the same way. Nor do they share the same natural oscillation periods; absent visual cues, the periods of the average person’s SCN neurons would be distributed around an average value slightly longer than a day— about 24.5 hours.
The article is based on a paper Resynchronization of circadian oscillators and the east-west asymmetry of jet-lag, Chaos 26, 094811 (2016) by Zhixin Lu, Kevin Klein-Cardeña, Steven Lee, Thomas M. Antonsen, Michelle Girvan, and Edward Ott. The paper is behind a paywall and my library does not subscribe to it but here is the abstract.
Cells in the brain’s Suprachiasmatic Nucleus (SCN) are known to regulate circadian rhythmsin mammals. We model synchronization of SCN cells using the forced Kuramoto model,which consists of a large population of coupled phase oscillators (modeling individual SCN cells) with heterogeneous intrinsic frequencies and external periodic forcing. Here, the periodic forcing models diurnally varying external inputs such as sunrise, sunset, and alarm clocks. We reduce the dimensionality of the system using the ansatz of Ott and Antonsen and then study the effect of a sudden change of clock phase to simulate cross-time-zone travel. We estimate model parameters from previous biological experiments. By examining the phase space dynamics of the model, we study the mechanism leading to the difference typically experienced in the severity of jet-lag resulting from eastward and westward travel.
Their model predicts that the worst cases of jet lag occur when traveling east across nine time zones and that recovery times can take as long as six days, though in my case I am able to adjust to the new time zone usually within about 24 hours. It turns out that Sri Lanka is on average 10 time zones east of Cleveland, which is unfortunate for Sri Lankans living in the US who go back for visits.